The present invention relates to an ink jet print head of the type set forth in the introductory part of claim 1.
Ink jet heads have been proposed, which produce an ink jet from a nozzle, due to instantaneous vaporisation of a portion of ink in the nozzle so as to expel the layer of ink thereon. In one known head, the ink is conductive and submerges a pair of electrodes which are disposed in facing relationship in a common plane that is perpendicular to the nozzle, whereby vaporisation is caused by passing current between the two electrodes through the ink. Such a head suffers from the disadvantage of requiring the nozzle to be of relatively large diameter so that it is not possible to print sufficiently small dots for high-definition printing.
In another known head, a resistive element which is disposed in the nozzle is energised in such a way as to transmit heat to the ink which is disposed around it. This suffers from the disadvantage of the thermal inertia in regard to transmission of the heat, so that the printing rate is relatively low. The head is also complicated to manufacture and suffers from a low level of reliability by virtue of the operating life of the resistance element.
A head has also been proposed, wherein the ink is conductive and in contact with an electrode which is disposed within the container while the counterelectrode is formed by a metal layer on the outside surface of a plate which carries the nozzle. The ink is ejected primarily by the vaporisation produced in a restricted section of the nozzle by the density of electrical current passing through the ink. This head makes it possible to print dots formed by a plurality of droplets which rapidly dry, but it requires the paper to be held very close to the nozzle and at a strictly constant distance therefrom.